Yb-Yb correlations and crystal-field effects in the Kondo insulator YbB12 and its solid solutions

We have studied the effect of Lu substitution on the spin dynamics of the Kondo insulator YbB12 to clarify the origin of the spin-gap response previously observed at low temperature in this material. Inelastic neutron spectra have been measured in Yb1-xLuxB12 compounds for four Lu concentrations x = 0, 0.25, 0.90 and 1.0. The data indicate that the disruption of coherence on the Yb sublattice primarily affects the narrow peak structure occurring near 15-20 meV in pure YbB12, whereas the spin gap and the broad magnetic signal around 38 meV remain almost unaffected. It is inferred that the latter features reflect mainly local, single-site processes, and may be reminiscent of the inelastic magnetic response reported for mixed-valence intermetallic compounds. On the other hand, the lower component at 15 meV is most likely due to dynamic short-range magnetic correlations. The crystal-field splitting in YbB12 estimated from the Er3+ transitions measured in a Yb0.9Er0.1B12 sample, has the same order of magnitude as other relevant energy scales of the system and is thus likely to play a role in the form of the magnetic spectral response.Comment: 16 pages in pdf format, 9 figures. v. 2: coauthor list updated; extra details given in section 3.2 (pp. 6-7); one reference added; fig. 5 axis label change

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Unification modulo ACUI plus Homomorphisms/Distributivity

E-unification problems are central in automated deduction. In this paper, we consider theories that are extensions of the well-known ACI or ACUI, obtained by adding finitely many homomorphism symbols, or a symbol `#' that distributes over the ACUI- symbol denoted `+'. We first show that when we adjoin a set of commuting homomorphisms to ACUI, unification is undecidable. We then consider the ACUID l -unification problem, i.e., unification modulo ACUI plus left-distributivity of a given `#' w.r.t. `+', and prove its NEXPTIME-decidability. When we assume the symbol `#' to be 2-sided distributive w.r.t